'\" et
.TH FTRUNCATE "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
ftruncate
\(em truncate a file to a specified length
.SH SYNOPSIS
.LP
.nf
#include <unistd.h>
.P
int ftruncate(int \fIfildes\fP, off_t \fIlength\fP);
.fi
.SH DESCRIPTION
If
.IR fildes
is not a valid file descriptor open for writing, the
\fIftruncate\fR()
function shall fail.
.P
If
.IR fildes
refers to a regular file, the
\fIftruncate\fR()
function shall cause the size of the file to be truncated to
.IR length .
If the size of the file previously exceeded
.IR length ,
the extra data shall no longer be available to reads on the file. If
the file previously was smaller than this size,
\fIftruncate\fR()
shall increase the size of the file. If the file size is increased,
the extended area shall appear as if it were zero-filled. The value of
the seek pointer shall not be modified by a call to
\fIftruncate\fR().
.P
Upon successful completion, if
.IR fildes
refers to a regular file,
\fIftruncate\fR()
shall mark for update the last data modification and last file
status change timestamps of the file and the S_ISUID and S_ISGID bits
of the file mode may be cleared. If the
\fIftruncate\fR()
function is unsuccessful, the file is unaffected.
.P
If the request would cause the file size to exceed the soft file size
limit for the process, the request shall fail and the implementation
shall generate the SIGXFSZ signal for the thread.
.P
If
.IR fildes
refers to a directory,
\fIftruncate\fR()
shall fail.
.P
If
.IR fildes
refers to any other file type, except a shared memory object, the
result is unspecified.
.P
If
.IR fildes
refers to a shared memory object,
\fIftruncate\fR()
shall set the size of the shared memory object to
.IR length .
.P
If the effect of
\fIftruncate\fR()
is to decrease the size of a memory mapped file
or a shared memory object
and whole pages beyond the new end were previously mapped, then the
whole pages beyond the new end shall be discarded.
.P
References to discarded pages shall result in the generation of a
SIGBUS signal.
.P
If the effect of
\fIftruncate\fR()
is to increase the size of a memory object, it is unspecified
whether the contents of any mapped pages between the old end-of-file
and the new are flushed to the underlying object.
.SH "RETURN VALUE"
Upon successful completion,
\fIftruncate\fR()
shall return 0; otherwise, \-1 shall be returned and
.IR errno
set to indicate the error.
.SH ERRORS
The
\fIftruncate\fR()
function shall fail if:
.TP
.BR EINTR
A signal was caught during execution.
.TP
.BR EINVAL
The
.IR length
argument was less than 0.
.TP
.BR EFBIG " or " EINVAL
.br
The
.IR length
argument was greater than the maximum file size.
.TP
.BR EFBIG
The file is a regular file and
.IR length
is greater than the offset maximum established in the open file
description associated with
.IR fildes .
.TP
.BR EIO
An I/O error occurred while reading from or writing to a file system.
.TP
.BR EBADF " or " EINVAL
.br
The
.IR fildes
argument is not a file descriptor open for writing.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
None.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIopen\fR\^(\|)",
.IR "\fItruncate\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<unistd.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
